Casting technology'.ppt

م�ن عق�د�ة� لل� و�اح�ان�ي {27}ل�س�

ق�و�ل�ي �ف�ق�هوا {28}ي

ح�من� �الر الله� � م �س� ب

�� ح�يم � الر

ا العظيم صدق

Casting Technology

The purpose of the casting procedure is to provide exact metallic duplicate of missing tooth (or teeth) structure with

maximum possible accuracy in all dimensions, For →

InlaysOnlays

Crowns & Bridges

Direct restoration

Indirect restoration

InlayOnlay

Gold bridge

Reduced tooth

Impression

Cast / Die

Gold Crown

Steps of casting procedures

Wax pattern constructionSpruingInvestingWax eliminationCastingFinishingPolishing

I- Wax pattern construction

Direct technique

Indirect technique

Indirect direct technique

II- Spruing

Create a channel to allow the molten wax to escape from the mold.

Enable the molten alloy to flow into the mold which was previously occupied by the wax pattern.

Role of a sprue:

The wax pattern is attached to a sprue

for proper handling without distortion

Sprue materials

Waxes

melt at the same rate as the wax pattern thus allowing easy escape of the molten wax

Plastics

MetalsHollow

Or

solid

Attachment and Direction: Sprue should be attached to the

thickest part of the wax pattern

with a 45º C angle to the proximal wall

This orientation avoids:

* Turbulent flow of the molten metal

→ air bubbles leading to

# porosity

Diameter (Thickness):

Equal or slightly Thicker than the thickest part of the wax pattern →

* improves flow of alloy into the mold • provides reservoir during solidification.

Reservoir: piece of wax attached 1 mm. Away from the wax pattern it is the last part to solidify,allows continuous feeding of molten alloy to compensate solidification shrinkage and to avoid shrinkage porosity

Number of spruesmay be single, double or multipleThere may be a main sprue with accessory sprues.

Single Double Multiple

Length of sprueShould keep the wax pattern

about 6-8 mm from the end of the casting ring

Short sprue → prevent escape of air leading to

back pressure porosity.Long sprue → fracture of the investment, as mold will not withstand the impact force of the entering molten alloy.

Preparation of the casting ring

Crucible former: usually made of rubber, which constitutes the base of the casting ring during investing.

Casting ring and liner: Casting ring may be metallic, or rubber ring.

Selection depends on: choice of casting alloy and investment material.

Casting ring lined using asbestos,

then wet the liner.

Functions of liner:

1) Creates space to allow for investment expansion.

2) Allow setting of investment material under water to give hygroscopic expansion.

3) Facilitate removal of investment from the casting ring after casting.

Types of ring

Metallic Rubber Split

III- Investing

Surfactant

Investing the wax pattern:Wax pattern must be invested immediately to avoid distortion of wax pattern,

which may be due to release of internal stresses

Wax is a hydrophobic material → Improper wetting of wax pat. →

irregular surface of the metallic cast. Minimized by applying

surfactants (surface acting agents)

on the wax surface → * increases surface energy of the wax * decrease contact angle between and

investment material.

Methods of investing the wax pattern: Hand investing procedure and vacuum ***Vacuum mixing of investment

is highly recommended to obtain casting with minimal surface defect.

Investing wax pattern may be

done by regular mix investment technique OR by double mix investment technique → * improve adaptability of the investment material to the surface of wax pattern * avoid distortion of wax

# After the ring is filled to the rim, # allow the investment to set for about one hour # remove the crucible former and the metallic sprue

Investing technique

Hand mixing Vaccum

IV- Wax elimination

Wax elimination: Heating the investment must be done

to achieve the following:1) All traces of the wax are vaporized: bring

the furnace to 200º C and hold this temperature

for 30 minutes . Most of the wax will be eliminated by this

time.2 )Increase the temperature gradually to the

final burnout temperature (about 650º C), this is accompanied by thermal expansion of the investment.

The mold temperature is now ready for casting.

V- CastingA) Melting the alloy

Using flame Using electric furnace

B) Casting:A casting machine requires * a heat source to melt the alloy. * casting force, so the casting will be completed.

Methods of melting alloys:* flame: Melting gold alloys is done by a blow pipe flame while melting base

metals is done by oxyacetylene flame.

* electric furnace.Methods of Casting: (driving the alloy into the mold)* Air pressure casting machine Compressed air* Centrifugal casting machine.* Both centrifugal and gas pressure

Casting machines

Air pressure casting machine Centrifugal casting machine

Solidification of the alloy is done under pressure, then

Allow for cooling and

when the color of the molten alloy has changed

from red to black color,

the ring should be quenched in water.

Devesting , Finishing and polishing of the casting are subsequently performed.

VI- Divesting and finishing

VI- Polishing

Casting Defects

I) Distorted casting: Due to distortion of the wax pattern.

** Improper handling of the wax pattern and/or its

** Delayed investing.

II) Dimensional inaccurate casting: Small or large casting may be due to :

a. Improper type of investment material.

b. Improper temperature.

III) Surface roughness of the casting: due to:

*** Air bubbles on the surface of wax pattern during the investment procedure

* Improper use of wetting agent

* Improper vacuum investing

VI) Porosity:

1) Shrinkage spot porosity:

at the point of attachment to the sprue, it is due to using a

thin sprue,

which leads to incomplete feeding of the molten alloy during solidification.

2) Back pressure porosity: It is due to using

***short sprue. The distance between the wax pattern and the end of the ring is more than 6 mm;

***Non porous investmentair will not escape from the mold and creates back

pressure during flow of molten alloy into the mold.

V) Incomplete casting and rounded margin:

Due to resistance of the flow of molten alloy into the mold:

*Using short sprue.

*Insufficient venting of the Investment.

*Insufficient casting pressure.

*Under heating of the alloy causing premature solidification.

*Incomplete wax elimination.